The present invention relates to a liquid level control system, and more particularly to a system for controlling the level and pressure of a cryogenic fluid that is dispensed from a main reservoir to a cooling enclosure.
A very simple liquid level control is shown in U.S. Pat. No. 4,745,760 by the present inventor which issued May 24, 1988. This patent is commonly assigned to NCR Corporation, which has its headquarters located in Dayton, Ohio, and the disclosure of this patent is hereby incorporated by reference. The fluid flow system described in this patent is essentially a free flow, static system in which the level of fluid in a main reservoir equalizes with the level of fluid in the cooling enclosure according to Pascal's law. However, since the gaseous evaporated fluid is partially restricted from leaving the cooling enclosure in order to guide it back into the outer portion of the fluid transfer conduit, a slight back pressure exists in the cooling enclosure which causes its fluid level to be slightly lower than the fluid level in the main reservoir. This simple system has no provision for filling the main reservoir to a level substantially above that of the cooling enclosure, or for turning off the flow of the cryogenic fluid from the main reservoir to the cooling enclosure should that be necessary.
Turning off the flow of the cryogenic fluid is not a minor task. Cryogenic conduits, such as the ones described in the above referenced patent, have greater heat losses than well insulated reservoirs or enclosures. This fact, coupled with the fact that a cryogenic fluid expands tremendously when its temperature reaches the vaporization point, clearly indicates that some provision must be made to clear the conduit of the cryogenic fluid before it vaporizes whenever the fluid supply from the main reservoir is turned off.
The capability to turn off the flow of the cryogenic fluid is an important one because it simplifies the task of filling or changing the main reservoir. The capability to turn off the flow of the cryogenic fluid also allows for maintenance or protracted inactivity of the system.
As mentioned above, the cryogenic fluid vaporizes if left in the cryogenic conduit. Vaporization would create a marked increase of the pressure within the cryogenic conduit which would require a pressure relief of some sort. Simply draining the conduit into a waste receptacle is a considerable waste of energy. Further, draining the conduit back into the main reservoir risks the introduction of heat and contamination into the main reservoir. Contamination, especially oxygen from the atmosphere, can be particularly dangerous. Oxygen liquifies at cryogenic temperatures and, since it has a different density than liquid nitrogen or liquid helium, collects in the system. Liquid nitrogen and liquid helium are relatively inert chemically, but liquid oxygen may explode into flame upon contact with an oxidizable material.
It is an object of this invention to provide a system for controlling the dispensing of a cryogenic fluid from a well insulated main reservoir to an insulated component mounting enclosure.
It is another object of this invention to provide a system for controlling the dispensing of a cryogenic fluid from a main reservoir to an insulated component mounting and for controlling the draining of a cryogenic conduit into a control reservoir after the main reservoir flow is turned off.
It is another object of this invention to provide a device for reducing the contamination of the cryogenic fluid drained into the control reservoir after the main reservoir flow is turned off.